% conclusion.tex

\chapter{Conclusion and Outlook}

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%resume + improvements

%generally
The system presented in this thesis has the potential of a full-fledged application. The analysis of the requirements, as well as the design of the system architecture, was done using common and well proven software engineering techniques. Whereby, the balancing act between the reutilization and the easy interchangeability in opposite to the high application awareness  succeeded for the most parts. Notwithstanding the not entirely satisfying results and the open questions, the design is reasoned and self contained. 
 
%filter
\paragraph{}
In order to connect the microphone to the MSB a sensor module was created. The circuit covers a fourth-order Butterworth low-pass filter and an amplifier. Furthermore, the signal is transformed from bipolar to unipolar. The comparison of the measured frequency response and the expected theoretical response shows acceptable results. A direction for future work is to design a digitally controlled amplifier. This would open the possibility to remotely adjust and fine tune the event detection on hardware level. In the same way, the problem of a missing infrasonic reference source for the best suiting amplifying factor could be avoided. 

%synchronization
The time synchronization was one of the big challenges. I showed the feasibility of achieving the required accuracy by a rare use of the GPS device. The implemented approach comprises a high potential to get improved. In view of the measurement results, the prediction of the crystal oscillator frequency by taking the current temperature into account is promising. Even the usage of an external and more stable oscillator would be an enhancement. The maximum achievable accuracy is determined by the GPS receiver and lies in the range of microseconds.   

%aquisition
The time synchronization greatly benefits the data acquisition. On the one hand, the synchronized hardware timer controls the sampling frequency through the pulse-width modulation, what assures the desired sampling frequency. On the other hand, the timer determines the beginning of the sampling. So, the mapping of the samples to the UTC time is reliably done.

The conflict of the high CPU demands of both, the data acquisition and the data transmission, is solved by applying a mutex for the CPU. Anyhow, considering a long-term usage it is recommended to implement a hardware trigger and start the data acquisition only if a threshold is passed. Therefore, the requirement of the signal opening needs to be discarded. A trade-off is the continuously sampling by just a single node.

The presented strategies for the local and the distributed event detection can be applied and expediently tested firstly, when several infrasonic sensor nodes are available. Again, the missing of an adequate reference signal complicates an contingently ongoing development. Future work should concentrate on the remote parameterizing of the thresholds. The event detection is an own field of research. An improvement here would payoff by reducing the power consumption of the network.     

%routing + gateway
Another big challenge was the implementation of the routing protocol in respect to the time slots. The procedure was very time consuming and the test results didn't show a sufficient behavior. Anyhow, I showed the functioning in principle and submitted possible adjustments. The most promising approach is the scanning of the time slots in the case of asynchrony. The benefit of the sleep scheduled radio for almost 97\% of the time still reasons such an approach. A detailed analyze and proving of the routing protocol can fill an own study.

%vdm
The implemented concept of the virtual data memory was highly optimized to the demands of the accessing components. The SD card read/write operations are only proceeded when it is absolutely required. For the data acquisition a free memory block is always assured. The Network component finds enough free memory in the reception time slot, while in the transmission time slot the memory is filled by the packets to be forwarded. In order to serve gateway requests of missing packets a history maps the packets to SD card addresses. The thereby required trade-off between the history size and the SD card address limitation can  be solved by considering the swapping out the entire history to the SD card.

%energy 
Finally, designing a circuit for the regeneration of chargeable batteries by solar panels could be the essential step towards a real long-term usage.

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% outlook
\paragraph{}
Wireless sensor networks present many exciting opportunities. The developed system is easily to customize in order to operate in different applications. For instance, the orthogonal usage of the infrasonic microphones can be used to measure the resonance frequency of high buildings. The implementation of a burglar alarm is another example, as well as tracing of moving objects by the time synchronized nodes. Nevertheless, exchanging the microphones to other sensors opens a broad spectrum of possibilities. 

%uav support

%resonance frequency

%burglar alarm

%radio activity



